A connecting rod that couples an engine crank shaft to a piston has one end as a small end, in which a small end hole pivotally supporting a piston pin is formed and the other end as a large end in which a large end hole pivotally supporting a journal portion of the crank shaft. Generally, in manufacturing, such a connecting rod is integrally molded by forging and is then broken into a rod portion and a cap portion at the large end.
Conventionally, as disclosed in Patent Documents 1 to 3, the connecting rod is broken by first forming a breakage-promoting wedge-shaped groove (notch) in an inner circumference of the large end hole. Then, as shown in FIG. 11, two-way split molds 50, 51 are inserted into a large end hole 53 of a connecting rod 52, and a wedge 54 is driven between the split molds 50, 51 to separate the split molds 50, 51 from each other, thereby causing breakage using as a starting point a notch 55 formed in the inner circumference of the large end hole 53.